There is a lot of useful information in the comments of the Default.ReaperOSC configuration file; it may be hard to wrap your head around all of it, though. TouchOSC is explained in detail on hexxler's website; I'm not familiar with OSC Commander at all, so can't help you with that one.

What is needed to make REAPER and the OSC client 'talk' to each other in a meaningful way, is that they use messages that both of them understand. That means that you either (1) have to change the message patterns in a *custom* .ReaperOSC configuration file (as editing the Default.ReaperOSC file has no effect whatsoever!) to define what message patterns REAPER understands, or (2) change the configuration used in the OSC client (TouchOSC, OSC Commander, etc.) to match the message patterns used in the .ReaperOSC configuration you're using (which may be the Default, or some custom configuration file).

I would recommend making a copy of the Default.ReaperOSC configuration file, renaming it, and editing it where needed, while sticking to the patterns used in the Default.ReaperOSC configuration as much as possible. Then edit the configuration for the OSC client to match those patterns. There are layout editors for TouchOSC available on hexxler's website. However, iirc, there were some issues with loading custom layouts to the Android version of TouchOSC a while ago, and I don't know if that has been changed yet. If not, your best bet may be to use one of the layouts that come with TouchOSC, and using a .ReaperOSC configuration file that matches it - iirc, there are files on both ends for a configuration/layout based on Logic's default configuration. Those may work, but I will warn you that the message patterns used by them may be much harder to understand - I found them very clumsy and misleading - and thus not very useful for learning how to use REAPER's OSC Control Surface feature at all.

Thanks, Geoff! Right now I am really only using TouchOSC to rtigger recording remotely from my drum kit, but I am keen to see if I can eventually replace the stuff I have programmed into my old Korg Nano Kontrols with OSC equivalents. Looks like your project could end up being the way to go. I have struggled all along to follow what you are developing (well above my pay grade) but cant wait to see it morph into something dummies like me can use.

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I’m in the process of trying to set up my custom OSC layout with CSI. I’m a bit stuck, there’s a lot of great documentation about setting up midi surfaces, but not much about OSC surfaces. Would using OSC with CSI enable me to get feedback from reaper when triggering custom actions?